This invention relates generally to optical data transmission and especially to the optical add/drop multiplexing used in an optical data transmission system.
The capacity of an optical fiber can be multiplied by increasing the number of different wavelengths to be conveyed in the fiber. This may be effectively implemented by Wavelength Division Multiplexing (WDM). In fiber optical systems, it is possible to use various WDM techniques, e.g. a unidirectional WDM technique or a bi-directional WDM technique. In the unidirectional technique, the WDM multiplexer at the transmitting end of the fiber combines the different wavelengths in the same fiber, while the WDM demultiplexer at the receiving end separates the different wavelengths from each other. In a bi-directional WDM system, information of different wavelengths is transmitted simultaneously in the same fiber in opposite directions. WDM systems using 4-20 different wavelengths for bit rates of 1-10 Gb/s a channel are available commercially, but in the near future systems of 32 and 40 wavelengths will also be commercially available. All different wavelengths travelling in the fiber can be amplified at the same time by using a linear optical fiber amplifier (OFA) in connection with the WDM technique. To use the resources as efficiently as possible, the amplifiers ought to be used at their full capacity in either direction.
Conventional telecommunications equipment uses only one optical signal, that is, at each end there are an optical transmitter and an optical receiver. In wavelength multiplexing, many such independent transmitter-receiver pairs use the same fiber, as was mentioned earlier, either in one direction or in two directions. On the output side of the system there is hereby an optical multiplexer, to the inputs of which are connected several optical conductors, in each of which a certain wavelength xcexn is transmitted. FIG. 1 illustrates a system including N parallel transmitter-receiver pairs. Each source of information modulates one optical transmitter, of which each produces light at different wavelengths xcex1 . . . xcexN (N greater than 1). The modulation bandwidth of each source is smaller than the interval between wavelengths, so that the spectra of modulated signals will not overlap. The signals produced by the transmitter are combined into the same optical fiber OF in a WDM multiplexer WDM1, which is an entirely optical (and often passive) component. The node of the optical bus may be an add/drop multiplexer (one or more; not shown in the figure), which through an optical fiber is connected with an entirely optical (and often passive) WDM demultiplexer WDM2, wherein a reverse operation is done on the multiplexing, in other words, every incoming wavelength xcexn is separated to its own optical conductor. Thereupon, each signal is detected at its own receiver. A narrow wavelength window in a certain wavelength range is made available to the different signals. A typical system including e.g. four transmitter-receiver pairs in parallel could be such, wherein the signals are within a 1550 nm wavelength range, so that the first signal is in a wavelength range of 1544 nm, the second at a wavelength of 1548 nm, the third at a wavelength of 1552 nm and the fourth at a wavelength of 1556 nm. Nowadays a 100 GHz (about 0.8 nm) multiple is becoming the de facto standard for the distance between wavelengths, and this is also recommended by the ITU-T, so it has a strong official position.
Optical add/drop multiplexing (OADM) means that in an optical network different wavelengths are conveyed in the same fiber through the network, so that in a node of the network certain desired wavelengths are added and certain wavelengths are dropped. There are two kinds of OADM architecture: 1) all wavelengths are demultiplexed and combined again, whereby some wavelengths are dropped and added while some bypass the equipment transparently, or 2) only drop wavelengths are demultiplexed and only add wavelengths are multiplexed. Complete demultiplexing will cause unnecessary losses for all wavelengths, which of course is not desirable. The OADM structures may be either dynamic or static.
Central parameters for the functioning of the OADM are the following:
Attenuation of bypassing wavelengths (which means those wavelengths, which are directed through the optical add/drop multiplexer transparently from the input gate to the output gate) as they travel through the OADM multiplexer.
Cross-talk from bypassing wavelengths to drop outputs.
Attenuation of drop wavelengths as they pass through the OADM to the drop outputs.
Cross-talk from drop wavelengths to the outgoing fiber.
Attenuation of add wavelengths as they pass from the add inputs through the OADM.
Cross-talk from add wavelengths to the drop outputs.
In an optical loop network only a certain amount of attenuation is permissible between transmitter and receiver. The attenuation is formed by the a)-d) sum of the following items:
a) add attenuation of the transmitting node,
b) drop attenuation of the receiving node,
c) bypass attenuation of all nodes to be bypassed,
d) attenuation of all conveying fiber links.
When item d) is determined by the locations of the connections to be set up, the a)-d) sum must be made sufficiently small in the loop, so that it will fit into the remaining attenuation margin. Besides attenuation, another significant matter is to make cross-talks sufficiently small in the nodes of the loop, for the reason that they would not significantly reduce the signal quality in the drop outputs.
FIG. 2a) shows a simple optical drop device, which is based on the Fiber Bragg Grating and on a three-gate circulator. The Fiber Bragg Grating is a grating which is made into an optical fiber for a chosen wavelength and which reflects back the concerned wavelength in the opposite direction. To other wavelengths the grating is transparent, and the grating will let these waves through the grating without reflecting them. The circulator directs the light arriving from gate 1 out through gate 2 and the light arriving from gate 2 out through gate 3. In the case shown in the figure, the grating reflects back the wavelength xcex3 and then drops it through gate 3. Other wavelengths xcex1, xcex2 and xcex4 will pass through the grating. It is possible on the same path to add the add/drop functionality, whereby the same wavelength, which was dropped, may again be added, e.g. by a coupler, as is shown in FIG. 2b).
FIG. 2c) shows another simple add/drop device, the Mach-Zehnder interferometer (MZI), which typically includes two 3 dB 2xc3x972 couplers and an optical path, which combine the output gates of the first coupler with the input gates of the second coupler. The first coupler divides the light wave arriving from either input gate (i1, i2) equally to two output gates of the concerned coupler. Of the sub-waves having a phase difference of xcfx80/2 after bypassing the coupler, each one propagates along its own path into the second coupler of the interferometer. With the aid of two couplers a complete cross-connection is formed, whereby the light wave connected from the top (bottom) input gate of the first coupler is obtained from the bottom (top) output gate of the second coupler. A complete cross-connection may also be implemented in such a way that on the optical paths combining the couplers there is a mirror or a grating reflecting the desired wavelength back in its direction of arrival, whereby the wave will pass twice through the same coupler (FIG. 2d). Hereby the wavelength connected from the first input gate and divided equally onto two optical paths will be reflected back to the same coupler and it will be connected out through the second input gate of the said coupler.
The optical transmission technique is being constantly developed in order to implement the lower levels of broadband network architectures as entirely optical systems, which would allow entirely optical relaying of high-capacity information flows (with the aid of optical cross-connection). E.g. optical loop networks are also well suitable for add/drop functions. The network can be flexibly configured according to the demands of traffic with the aid of add/drop devices, with which those wavelengths are chosen which are to be added/dropped. The type of signal to be used in the network may vary; the signal may be e.g. a Synchronous Digital Hierarchy (SDH) signal, a Plesiochronous Digital Hierarchy (PDH) signal or an Asynchronous Transfer Mode (ATM) signal.
Add/drop devices may be implemented e.g. in a WDM loop network including a number of N network nodes connected to each other by optical fiber links, so that they form a loop. There may be more than one optical fiber, and in each fiber several signals may travel with different wavelengths (xcex1, xcex2 . . . xcexN). Special loop cases are the one-fiber loop and the two-fiber loop. In a one-fiber loop, the signals usually travel in one direction in the loop, either clockwise or anti-clockwise. It is also possible to implement bi-directional traffic in the same fiber. In regard to bi-directional traffic it is usual that with each different wavelength only one direction is in use in one link. In consequence of this, a connection can be set up between two adjacent nodes by the shortest route directly through the link only in one direction, whereas in the connection of the opposite direction the signals have to circle along a longer route around the loop. In a two-fiber loop the above-mentioned problem does not occur. A connection can be formed between node pairs in two different circling directions. Hereby the signals will travel in two fibers in opposite circling directions in relation to each other and generally primarily using the shorter connection. Longer routes are standby routes in case of interference situations. For example, if the cable between the concerned nodes is damaged, the connection need not necessarily be cut off, since a longer route may then be used.
In the nodes of the loop add and drop signals with different wavelengths xcex1 . . . xcexN as well as signals bypassing the node are processed. The node adds an add signal to the WDM signal going to another node. The drop signal is a signal which the said node receives from another node and which is separated by the said node from the incoming WDM signal. The signals bypassing the node are signals between two other nodes which travel only through the node from a directly incoming fiber to an outgoing fiber, that is, the node directs the signals from the arriving WDM signal directly to the outgoing WDM signal. The signals are not changed into electric form and they are not processed electronically, but the OADM device carries out the necessary functionality entirely optically as regards the three different signal types mentioned earlier. An OADM device should usually be transparent to the signals. In practice, this means that it must be able within certain limits to deal with signals of different types that have e.g. different bit rates and other characteristics. E.g. add signals come to the device from telecommunications equipment located in the node. Drop signals again are directed from the OADM device to the telecommunications equipment in the node. The said equipment is not included in the OADM device, so the external signal processing performed by it is not essential from the viewpoint of this invention. More essential characteristics are mainly the wavelength and optical power of the signal.
Two special cases of loop network were studied above. A general case will be studied in the following which is a symmetrical loop of a complete connection. It is assumed that the network includes N network nodes and OADM devices located in these as well as optical fibers between the nodes. Hereby one optical fiber comes into each network node and one optical fiber goes out from each node. Thus, the loop network is a one-fiber loop, wherein bi-directional connections between all different node pairs are set up in one circling direction, either clockwise or anti-clockwise. One wavelength xcex1 is reserved for use by each node pair. Around the loop a connection can be set up between two node pairs along two different routes; on these routes bi-directional traffic is formed with the wavelength in question, whereby the reserved wavelength is used in all links of the loop. Hereby the concerned wavelength is both an add wavelength and a drop wavelength for each node of the node pair, but a bypassing wavelength for (Nxe2x88x922) nodes. As the number of nodes increases, the number of necessary wavelengths will also grow. At the same time, the proportion of wavelengths bypassing each node grows more than the proportion of add/drop wavelengths. When the number of nodes is N, the number of necessary wavelengths is the same as the number of various node pairs, that is, N(Nxe2x88x921)/2. The number of add/drop wavelengths in each node is the same as the number of other nodes, that is, Nxe2x88x921. The other wavelengths are bypassing wavelengths for the said node and their number is (Nxe2x88x922)*(Nxe2x88x921)/2. When two symmetrical one-fiber loops as described above, each with an opposite circling direction, are superimposed, a two-fiber loop is obtained which has standby connections.
Attenuation and dispersion are the most significant detrimental effects in optical data transmission. As the number of network nodes increases, attenuation will also grow. It is true that amplifiers may be added, but this is not a very desirable solutions, since amplifiers will add noise and also costs. At network level problems are also caused by the arrangement of a suitable wavelength specific amplification, because different wavelengths may travel along different routes, which may have quite big differences in length. Dispersion can be compensated for e.g. by the so-called Bragg chirped gratings. In this grating, the grating cycle varies linearly as a position function, owing to which the grating reflects different wavelengths from different points and will thus cause different delays for different frequencies. In a conventional fiber, dispersion will usually cause longer delays for low-frequency components. Chirped gratings are used explicitly to cause longer delays to components of higher frequencies and in this way to compensate for delay differences. In practical implementations the band must be very narrow, that is, a separate grating is chosen for each desired wavelength.
The invention is intended to bring about a solution by which drawbacks relating to attenuation can be reduced, while at the same time a network is implemented with a better cost efficiency.
The invention concerns optical add/drop multiplexing used in an optical data transmission system. It is an objective of the invention to bring about a solution reducing the drawbacks presented above, which relate to optical data transmission, mainly to attenuation and cost efficiency. The established objective is achieved in the manner presented in the independent claims.
The inventive idea is to implement an OADM device by using a group formed of two direction selective organs and of one or more wavelength selective organs located in between, by connecting several such groups together in a manner suitable for each node and by forming several add inputs and drop outputs of the gates of the direction selective organs, which input and output gates can be further selected so that any attenuation caused by the said device is optimised in the best possible way in view of the whole telecommunications network. Owing to the solution according to the invention, the configurations of the add/drop devices can be optimised as regards losses of those connections which are most critical (longest) as regards network attenuation, and in this way as small an attenuation as possible can be obtained for the concerned connections in the OADM device itself. The solution also provides a very systematic way of reducing the total attenuation at network level, which way is also versatile as regards its implementation alternatives, because the optimising can be carried out for one node at a time, both by choosing a device configuration which is suitable for the concerned node and by choosing, within the chosen configuration, such gates for use which are optimum gates in regard to the different wave lengths. Owing to the solution according to the invention, there is also less need than before for optical amplifiers which increase noise and costs.
The add/drop devices are implemented by using optical Fiber Bragg Grating as the wavelength selective organs and optical circulators as the direction selective organs. Alternatively, the individual group can be implemented with the aid of a Mach-Zehnder interferometer. The attenuation is optimised by choosing separately for each application a manner of arrangement of either gratings and circulators or, alternatively, Mach-Zehnder interferometers containing Fiber Bragg Gratings, for the transmission and add-drop connections.
In an advantageous embodiment of the invention chirped gratings for compensating for dispersion are used for those wavelengths which require compensation.
Using the optical add/drop multiplexing device according to the invention advantages are achieved compared with conventional solutions, the most important advantage being the possibility to optimise the OADM devices for the most critical connections and, secondly, the possibility to perform separately for each wavelength both dispersion compensation and reflection by using the grating.
In technical terms, the add/drop device in accordance with the invention can be implemented e.g. in a WDM loop network.